Proteolysis has multiple functions in the nervous system, and participates in such processes as protein turnover, formation and inactivation of neuropeptides, elimination of excess or damaged cells, and activation and inactivation of enzymes. Protein catabolism is altered during growth and differentiation, and in response to physiological and pharmacological alterations. Changes were observed in a number of pathological conditions, and there are indications that proteolysis is altered in aging and that such changes may be at lest in part responsible for pathological changes in senile dementia. A further understanding of protein catabolism during aging is crucial to our understanding of the aging process, of senile changes in proteins, and would contribute to our understanding of cerebral protein turnover. The present project will measure changes during aging in cerebral protein breakdown in vivo and in vitro. In vivo protein breakdown will be measured by measuring the decay of label of prelabeled proteins in several fractions (cellular, subcellular, and membrane fractions), prepared from several brain areas in male and female rats. For in vitro experiments, cathepsin D, Ca-activated neutral proteases, and a number of brain proteins representing structural, membrane, cell-specific proteins and enzymes will be purified, and the breakdown of the substrates by the enzymes will be measured with quantitative gel electrophoresis in a crossover design (testing adult and senescent enzyme on substrate from adult and senescent brain) in several brain regions. We will also test changes in total and soluble acid and neutral protease activity. Any specific area or fraction showing changes will be investigated in greater detail. The aim of the present phase of this study is to examine changes in vivo catabolism of selected brain protein fractions and changes in the properties of the proteases during aging. We hope to be able to identify specific proteins whose catabolism is altered and to indicate the changes involved in the properties of the substrates or enzymes. In further work we hope to study age- related changes in catabolism of other brain proteins and to study possible changes in other factors such as endogenous inhibitors, substrate phosphorylation, endocrine influences, etc.